Lightroom 5.6: MP3 audio file won't load in Slideshow Module

Lightroom 5.6 ver. will not load MP-3 audio in the Slideshow module. Re-installed to the original 5.2 ver. and this audio problem is still there. The MP-3 audio file is a tested and proven working file. Was able to load this file into my slideshow using Lightroom 4. at Tri-Community Photography school, So. Cal. Do you have a fix?

The OS ver. on my computer and the schools is Windows 7. Not sure about the codecs but I did an upgrade to mine with no change. Lightroom still does not recognize the MP3 file. This is Lightroom 5.2 I purchased on CD. I installed it on my Mac Book Air and it worked fine, no problem. The Mac and Windows installs are 2 separate files, correct?
Need help, please!